1. Field of the Invention
The present invention relates to a process for obtaining pure 1,3-butadiene from crude 1,3-butadiene by distillation and to an apparatus for carrying out this process.
2. Description of the Background
On an industrial scale, 1,3-butadiene is generally obtained from the C4 fraction, i.e. from a mixture of hydrocarbons in which the C4-hydrocarbons, in particular 1-butene, i-butene and 1,3-butadiene, predominate. Apart from small amounts of C3- and C5-hydrocarbons, the C4 fraction generally comprises butynes, in particular 1-butyne (ethylacetylene) and butenyne (vinyl-acetylene). Here, a crude 1,3-butadiene, i.e. a mixture comprising from about 89 to 99.5% by weight of 1,3-butadiene, remainder impurities, is obtained initially. In order to meet specifications, this subsequently has to be purified further by distillation to give pure 1,3-butadiene. The specifications for pure 1,3-butadiene provide, in particular, for a minimum 1,3-butadiene content of 99.6% by weight and a maximum permissible propyne content of 10 ppm and 1,2-butadiene content of 20 ppm.
Owing to the low differences in the relative volatilities of the components, the isolation of crude 1,3-butadiene from the C4 fraction is a complicated distillation problem and is therefore generally carried out by extractive distillation.
The acetylenic C4 impurities, in particular ethylacetylene and vinylacetylene, can also be particularly advantageously converted into the desired product 1,3-butadiene by carrying out a selective hydrogenation before the extractive distillation, for example as described in U.S. Pat. No. 4,277,313, or particularly advantageously by carrying out extractive distillation and selective hydrogenation over a heterogeneous catalyst in a single column, preferably a dividing wall column, or in thermally coupled columns. Such a process is described in the German Patent Application 10022465.2, which is not a prior publication and is hereby fully incorporated by reference into the disclosure of the present invention. However, the known processes for extractive distillation or extractive distillation and selective hydrogenation, for example as described in DE 10022465.2, initially give a 1,3-butadiene which does not yet meet specifications and is therefore referred to as crude 1,3-butadiene.
According to the prior art, purification of crude 1,3-butadiene by distillation to give pure 1,3-butadiene is carried out in two stages: In a first stage, a mixture of predominantly propyne and propadiene is taken off at the top of the column at a column pressure of about 7 bar, and in a second downstream distillation column, 1,2-butadiene and C5-hydrocarbons are separated off as bottom product at a pressure of about 4.5 bar. About half of the cis-2-butyne present in the crude 1,3-butadiene appears at the top of the second distillation column and about half appears at the bottom of this column. The desired product, namely pure 1,3-butadiene, is taken off at the top of the second distillation column.
EP-B 284 971 discloses thermally coupled operation of the two distillation columns. In the process of EP-B 284 971, too, the two distillation columns are operated at different pressures and thus each have to be equipped with their own vaporizer and condenser, resulting in only a slight reduction in energy consumption compared to the variant using two distillation columns which are not coupled thermally.
All known process variants for obtaining pure 1,3-butadiene from crude 1,3-butadiene by distillation started from the assumption that operation at two different pressures, with the pressure in the second distillation column being lower than that in the first distillation column, is absolutely necessary in view of the thermally sensitive dienes which tend to polymerize and also to achieve better condensability of the propyne/propadiene mixture at the top of the first distillation column.